Many devices and methods are known for use in exercising nearly all muscles of the human body. Most of these require the muscle being exercised to flex to operate the device. Many employ a spring or other device which acts in tension and require the user to pull the ends of the device apart to exercise the muscle.
Other forms of exercise are known which employ no assisting apparatus at all. For example, the sit-up is a well known exercise for developing the abdominal muscles.
The above forms of exercise are generally known as "dynamic" because the body parts are moving during the exercise. Another form of exercise, however, is known as "isometric" because the muscle is required to flex but is not required to move. For example, the position wherein one is supported with the hands while the feet are held in front of the body requires the abdominal muscles to be flexed while maintaining them stationary. This requires a significant amount of abdominal strength and is of value as an exercise only to those with such strength.